This invention relates to composite bonded permanent magnets having high flexibility and high magnetic particle loading.
Bonded magnets are manufactured from mixtures of magnetic powders and binding resins by pressure-molding the mixtures into desired magnet shapes. Of particular interest are the rare earth bonded magnets comprising a magnet powder containing a rare earth element or elements, which rare earth elements are generally understood to include elements 21, 39 and 57-71 of the periodic table of the elements. An exemplary rare earth magnet alloy is neodymium-iron-boron (Ndxe2x80x94Fexe2x80x94B).
The methods of manufacturing rare earth bonded magnets generally include mixing magnet material with a binder resin and forming the mixture into sheets, strips, or net shape parts by compaction molding, roll molding, injection molding and extrusion molding the mixture. In each of these processes, it is desirable to maximize the particle loading of the magnet material to provide optimum magnetic properties for the permanent magnet. In addition, it is desirable to provide a permanent magnet that is flexible. It has, however, been difficult to achieve a bonded magnet having both high magnetic properties and high mechanical flexibility.
In the various permanent magnet manufacturing methods, rapidly solidified, melt-spun ribbons of the magnetic material are comminuted into irregularly sized and shaped particles, specifically irregular flakes, which are then combined with the binder resin. Mixing methods may include calendar rolling or Banbury intensive mixing, for example. It is difficult to obtain high loading volume with very small flakes because it becomes increasingly difficult for the given volume of binder to wet the surface of the flakes as the particle size diminishes due to the intensive mixing process, so as to form a homogeneous and cohesive mixture. It has been observed that, after a certain loading is reached, the mixture tends increasingly to reject further particles, and the mixture becomes dry, crumbly, and loses adherence to further particles. Thus, larger, coarse flakes are used, but the large flakes interlock to an extent that harms homogeneity and mechanical flexibility. These large flakes also have a high tendency to fracture due to their brittle nature, such that the particle surface area increases. The binder matrix is weakened, and the composition then becomes dry and crumbly because the same amount of binder is present for coating an increased surface area.
In addition, with rare earth magnet material, the flakes easily oxidize if their size is reduced below a threshold value, causing the flakes to become pyrophoric and prone to spontaneous combustion if exposed to air even briefly. Due to the pyrophoric nature of the material, it has thus been considered necessary to incorporate large particles into the binder matrix. As previously stated, however, these large flakes have a tendency to fracture during the mixing and molding process, creating new surface area for oxidation and thereby creating the possibility for spontaneous combustion throughout the mixing and molding process. Also, upon fracture, binder is displaced, causing interference between flakes, and thus sparking within the mixture. Consequently, precautions must be taken due to the risk of sparking and fire, such as mixing of the flake particulate with the binder in an inert atmosphere. The need for precautions is particularly necessary in a batch process using a Banbury intensive mixer. Morever, as the flakes fracture, the magnetic properties drop dramatically.
In addition to the risk of fire and sparking due to fracturing of the flakes, coarse particles also tend to react adversely with and degrade in a wide range of polymer binder materials. Spontaneous pyrophoric and/or exothermic reactions with coarse NdFeB particles have occurred with various elastomers. While some reactions occur very suddenly, other mixtures slowly decompose, thereby compromising the long term stability of the rare earth bonded magnets. Some magnets have been limited to room temperature use due to poor heat aging.
Thus, there has been a need for a rare earth type permanent magnet having a high particle loading for optimum magnetic properties and high mechanical flexibility that is not highly pyrophoric during manufacture and which has long term high temperature stability, i.e., good heat aging.
The present invention provides a flexible permanent magnet in which atomized, generally spherical rare earth magnetic particles are bonded in a binder system including a nitrile rubber and precipitated amorphous silica. The bonded permanent magnets exhibit high mechanical flexibility and elasticity, good magnetic properties, and good heat aging. In addition, the magnet powder may be mixed with the binder with little to no risk of combustion. In an exemplary embodiment, a permanent magnet is provided comprising a nitrile rubber with about 23-37% acrylonitrile content, an ethylene vinyl acetate copolymer, a precipitated amorphous silica, and atomized, generally spherical rare earth magnet particles having a size distribution including a median particle size in the range of about 35-55 xcexcm with a standard deviation in the range of about 10-30 xcexcm and less than about 0.1% of the particles having a diameter above about 115 xcexcm, wherein the magnet has a percent ultimate elongation greater than about 100%.